The present invention pertains to pulse width discriminators and methods therefore and in particular to amplitude adjusted pulse width discriminators and methods therefor.
There are many applications in which it is desirable to select a particular pulse sequence from among a mixture of pulse sequences, each of which has a characteristic pulse width. For example, it is desirable to construct radar beacons which can distinguish between trains of interrogation pulses having one width and other radio frequency pulse trains having the same frequency but having pulses with a different width.
A large number of approaches have been applied to the problem of discriminating among pulses having different widths. However, these approaches ordinarily assume that the pulses to be distinguished have a common, sharp rise time and a common, sharp fall time. In reality, even where received pulses have a sharp rise time, the fall time is often degraded. Furthermore, commonly available video amplifiers decrease the sharpness of the rising and falling edges of received pulses even further.
One approach to pulse width discrimination involves the use of a fixed threshold comparator which is triggered when the rising edge of the pulse exceeds a threshold and when the falling edge of the pulse falls below the threshold. However, due to a lack of sharpness, commonly called pulse stretching, pulses which have the same width but which differ in amplitude trigger a fixed threshold comparator at different points on the pulse, resulting in an inaccurate indication of pulse width. For the case of radar interrogation beacons in a signal-crowded environment where the pulses to be distinguished differ only slightly in pulse width but may differ in amplitude as well, the fixed threshold approach is inadequate for discriminating between desired and undesired pulses similar in width and having nearly or exactly the same frequency.
A second commonly used approach involves subtracting a pulse from a delayed version of the same pulse and applying the resulting waveform to a window comparator having a threshold set at some fixed voltage above and below zero volts. Due to the fixed threshold, this approach is also degraded by the finite rise and fall times of the pulse.
A further approach that avoids errors due to rise and fall times depends upon the use of a logarithmic amplifier and upon the special shape of a log pulse. However most useful systems, require the use of a linear range for weak signals in order to prevent log amplification of noise. The log pulse width discriminator degrades in this linear range. This degradation is particularly undesirable because many other system functions are also degraded at weak signal levels.